代码之家 › 专栏 › 技术社区 › Dave

动态生成Linq lambda表达式

lambda c#

Dave · 技术社区 · 15 年前

好吧,我猜这已经在某个地方得到了答案,我只是对语法还不太熟悉,还不能理解,所以请耐心等待。

我的web应用程序的用户需要在一个通过linqdata源访问的gridview中过滤一长串项目。我正在使用OnSelecting事件进一步筛选项目。我想根据用户在DropDownlist中所做的选择筛选那些项目。

例如,他们选择“标题”包含“fred” 这导致

e.Result = dbContext.Opps.Where(opp => opp.Title.Contains("Fred"));

或“description”不包含“alpha” 结果在

 e.Result = dbContext.Opps.Where(opp => !opp.Description.Contains("Alpha"));

我希望动态构建该表达式(system.linq.expressions.expression>),而不是使用嵌套的switch表达式来生成它,因为有许多字段需要检查,我还希望使用startswith和endswith检查。如果我可以将表达式构建为字符串,比如:

string stringExpression = string.Format("opp => opp.{0}.{1}(\"{2}\")",
    ddlCustomFilter.SelectedValue,
    ddlFilterType.SelectedValue,
    txtFilterText.Text);

然后把它转化成一个表达式…这有可能吗?或者,我应该咬紧牙关生成创建各种表达式所需的所有switch()语句吗?

2 回复 | 直到 15 年前

tvanfosson 15 年前

当然可以动态地构建表达式,但我会考虑使用 Dynamic LINQ 作为备选方案,首先,尽管您可能无法使用contains。此外,您可以考虑使用 PredicateBuilder 以添加方式构建复杂查询。

suneelsarraf 11 年前

尝试此代码…

调用ToExpression方法()…

public static Expression<Func<T, bool>> ToExpression<T>(string andOrOperator, string propName, string opr, string value, Expression<Func<T, bool>> expr = null)
    {
        Expression<Func<T, bool>> func = null;
        try
        {
            ParameterExpression paramExpr = Expression.Parameter(typeof(T));
            var arrProp = propName.Split('.').ToList();
            Expression binExpr = null;
            string partName = string.Empty;
            arrProp.ForEach(x =>
            {
                Expression tempExpr = null;
                partName = partName.IsNull() ? x : partName + "." + x;
                if (partName == propName)
                {
                    var member = NestedExprProp(paramExpr, partName);
                    var type = member.Type.Name == "Nullable`1" ? Nullable.GetUnderlyingType(member.Type) : member.Type;
                    tempExpr = ApplyFilter(opr, member, Expression.Convert(ToExprConstant(type, value), member.Type));
                }
                else
                    tempExpr = ApplyFilter("!=", NestedExprProp(paramExpr, partName), Expression.Constant(null));
                if (binExpr != null)
                    binExpr = Expression.AndAlso(binExpr, tempExpr);
                else
                    binExpr = tempExpr;
            });
            Expression<Func<T, bool>> innerExpr = Expression.Lambda<Func<T, bool>>(binExpr, paramExpr);
            if (expr != null)
                innerExpr = (andOrOperator.IsNull() || andOrOperator == "And" || andOrOperator == "AND" || andOrOperator == "&&") ? innerExpr.And(expr) : innerExpr.Or(expr);
            func = innerExpr;
        }
        catch { }
        return func;
    }

    private static MemberExpression NestedExprProp(Expression expr, string propName)
    {
        string[] arrProp = propName.Split('.');
        int arrPropCount = arrProp.Length;
        return (arrPropCount > 1) ? Expression.Property(NestedExprProp(expr, arrProp.Take(arrPropCount - 1).Aggregate((a, i) => a + "." + i)), arrProp[arrPropCount - 1]) : Expression.Property(expr, propName);
    }

    private static Expression ToExprConstant(Type prop, string value)
    {
        if (value.IsNull())
            return Expression.Constant(value);
        object val = null;
        switch (prop.FullName)
        {
            case "System.Guid":
                val = value.ToGuid();
                break;
            default:
                val = Convert.ChangeType(value, Type.GetType(prop.FullName));
                break;
        }
        return Expression.Constant(val);
    }

    private static Expression ApplyFilter(string opr, Expression left, Expression right)
    {
        Expression InnerLambda = null;
        switch (opr)
        {
            case "==":
            case "=":
                InnerLambda = Expression.Equal(left, right);
                break;
            case "<":
                InnerLambda = Expression.LessThan(left, right);
                break;
            case ">":
                InnerLambda = Expression.GreaterThan(left, right);
                break;
            case ">=":
                InnerLambda = Expression.GreaterThanOrEqual(left, right);
                break;
            case "<=":
                InnerLambda = Expression.LessThanOrEqual(left, right);
                break;
            case "!=":
                InnerLambda = Expression.NotEqual(left, right);
                break;
            case "&&":
                InnerLambda = Expression.And(left, right);
                break;
            case "||":
                InnerLambda = Expression.Or(left, right);
                break;
            case "LIKE":
                InnerLambda = Expression.Call(left, typeof(string).GetMethod("Contains", new Type[] { typeof(string) }), right);
                break;
            case "NOTLIKE":
                InnerLambda = Expression.Not(Expression.Call(left, typeof(string).GetMethod("Contains", new Type[] { typeof(string) }), right));
                break;
        }
        return InnerLambda;
    }

    public static Expression<Func<T, object>> PropExpr<T>(string PropName)
    {
        ParameterExpression paramExpr = Expression.Parameter(typeof(T));
        var tempExpr = Extentions.NestedExprProp(paramExpr, PropName);
        return Expression.Lambda<Func<T, object>>(Expression.Convert(Expression.Lambda(tempExpr, paramExpr).Body, typeof(object)), paramExpr);

    }
    public static IQueryOver<T, T> OrderBy<T>(this IQueryOver<T, T> Collection, string sidx, string sord)
    {
        return sord == "asc" ? Collection.OrderBy(NHibernate.Criterion.Projections.Property(sidx)).Asc : Collection.OrderBy(NHibernate.Criterion.Projections.Property(sidx)).Desc;
    }

    public static Expression<Func<T, TResult>> And<T, TResult>(this Expression<Func<T, TResult>> expr1, Expression<Func<T, TResult>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, TResult>>(Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
    }

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>(Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
    }